It is a frequent complaint of users of certain types of topically applied products that they are irritating to the skin. This complaint is often associated with the emulsion-based products which typically utilize relatively large quantities of emulsifiers or surfactants to maintain the stability of the composition. Although there may be many reasons for this irritation, at least part of this problem may be connected with the action of the emulsifiers on the skin's native lipid barrier. The lipid barrier is a collection of varied lipids, such as triglycerides, ceramides, and free fatty acids, located in the stratum corneum. The barrier serves to prevent the penetration of most substances to the lower layers of the skin, as well as preventing water loss from the skin. Unfortunately, the very properties that make standard emulsifiers useful in maintaining a stable emulsion, i.e., the ability to couple hydrophilic and lipophilic materials, also potentially cause a disruption of the lipid barrier; this action in turn can permit more rapid penetration of any irritants that may be in the vicinity of the area where the barrier has been weakened. Thus, while the emulsifiers themselves are not necessarily irritants, their action on the skin can indirectly lead to irritation.
The problem is compounded when a topically applied emulsion contains an active or therapeutic component which is in itself irritating, even in the absence of emulsifiers. In such a case, the irritant is essentially being deliberately applied in combination with components which will almost certainly weaken the barrier, thereby permitting a rapid penetration of the irritant to the lower layers of the skin. This can potentially lead to a very significant level of discomfort to the user, which may cause the user to ultimately discontinue use of an otherwise therapeutically useful product.
The complete elimination of emulsifiers from topically applied products is difficult, if not impossible, to achieve, given the popularity and utility of emulsions as a delivery vehicle. However, as the foregoing discussion shows, their use in a composition with potentially irritating actives is problematical. Thus, there is a continued effort to find ways of formulating such actives without enhancing their irritation potential, and further, to reduce their irritation potential to levels which will be acceptable to the average user of the product. The present invention provides such formulations.